Deep root biomass and bNPP of a coffee agroforestry system in Costa Rica

Costa Rican coffee (Coffea arabica) plants are often grown in agroforests. Studying the relationship between shade-inducing trees on coffee plant biomass and root competition in the topsoil, and overall belowground net primary productivity (bNPP), Defrenet et al. estimate root biomass and bNPP at the stand level, taking into account deep roots and the positioning of coffee plants in relation to trees.

Frequency histogram of basal area at root collar (BAC, cm2) from an inventory at 697 locations (white bars) and for the 12 coffee plants chosen for root excavation and growth ring analyses (black bars). The selected plants for excavation covered 97% of the inventory range. Lines indicate lognormal distributions fitted to the inventory frequencies (solid line) and the excavated plants (dotted line) using maximum likelihood. The BAC of excavated coffee plants possessed the same distribution as that from plants in the inventory (P = 0·84 and P = 0·55, respectively).

Coffee root systems are shown to comprise 49% of the total plant biomass; such a high ratio is possibly a consequence of shoot pruning. There was no significant impact of shade-inducing trees on the fine root biomass (2.3 t ha-1), suggesting that coffee root systems are very competitive in the topsoil.

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